Environmental assessment of a hybrid system composed of solid oxide fuel cell,gas turbine and multiple effect evaporation desalination system

Abstract

This study deals with a solid oxide fuel cell- gas turbine (SOFC-GT) hybrid system coupled with a multi-effect evaporation desalination plant with steam condensation. The environmental evaluation is also done due to the importance of waste energy recovery especially waste heat in power generation systems. The evaporation desalination plant is studied for using the excess heat to produce freshwater. The thermodynamic relationships governing different components of the system are first provided, including fuel cells, heat exchangers, gas turbine, and desalination plant. Next, given the absence of previous research on the environmental effects of cogeneration systems, despite its necessity, the study system is analyzed from an environmental point of view. Accordingly, the impacts of the system performance parameters, including the fuel consumption coefficients, compressor pressure ratio, fuel pre-reforming percentage, and the steam to carbon ratio are investigated on the CO₂, CO, and NOx emission rates. Based on the findings, it is concluded that of different species, the impacts of CO, CO₂, and NOx emission rates are significant on the environment. Thus, the impacts of pressure ratio and pre-reforming percentage on their emission rates have been studied. The results revealed with increasing the compressor pressure ratio, increasing the fuel consumption coefficients, and decreasing the fuel cell's exhaust temperature, the CO and NOx emission rates and corresponding social costs diminished. On the other hand, with elevation of the ratio of steam to carbon, the recovery rate, the fuel cell's exhaust temperature, the concerned gas emission rates, and corresponding social costs increased.

Keywords

Solid oxide fuel cell gas turbine multi-effect evaporation desalination plant environmental investigation

Introduction

The use of fossil fuel-based energy sources in various industrial, domestic, and commercial sectors causes release of some toxic and harmful gases into the environment and has adverse effects on living things and nature. Pollutant emissions during the exploration, extraction, exploitation, transmission, conversion, distribution, and consumption of different energy carriers result in water pollution, air pollution, noise pollution, and soil pollution with each causing destructive effects on living things and the environment. The most important part of the energy sector related to air pollution is the use of fossil fuels as sulfur oxides (SOx), nitrogen oxides (NOx), carbon monoxide (CO), aerosols, and carbon dioxide (CO₂) are the major pollutants stemming from these fuels released into the atmosphere. Indeed, environmental issues may cause some climate change to occur in some parts of the world because of high-level pollutants generated by the combustion of fossil fuel. Note that the CO₂ emission rate has increased over the past 200 years and is still growing. Indeed, prior to the industrial revolution in the 19th century, CO₂ concentration was around 200 parts per million (ppm), while it reached more than 412 ppm in 2020.^1,2

According to studies, 76.9%, 34.7%, and 9.7 of total CO₂, methane, NOx emitted into the air are due to the fossil fuels consumed in the energy sector.³ In 2015, electricity and heat production, as well as transportation accounted for about two-thirds of CO₂ emissions in the world. Based on the estimations, electricity and heat production accounts for 42% and transportation for 24% of this share.³ Despite major growth in renewables energies, electricity and heat production is still mainly dependent on coal, a high-carbon fuel, with some countries, including Australia, China, India, Poland, and South Africa supplying more than two-thirds of their required electricity and heat from coal.³

In Iran, energy consumption has increased by about 188% from 1980 to 2000, while carbon emissions have increased by 142% accounting for 1.3% of the world's total carbon emissions.⁴ In 2006, the transportation sector had the largest share not only in the CO₂ emission but also in the production of other pollutants and greenhouse gases. The power plant, domestic/commercial/public, and transportation sectors claimed 29.3%, 25.4%, and 23.8% of CO₂ emissions among the power consumption and production sectors. Natural gas is considered as a clean fuel with lower pollution compared to other fossil-based fuels, however, 64% of total CO₂ emissions in the power sector is associated with natural gas.⁵ Figure 1 presents the CO₂, SO₂, and NOx emission rates from different power sectors in Iran in 2015.

Figure 1.

CO₂, SO₂, and NOx emission rates from different power sectors in 2015, Iran.⁵

Considering the issue of emissions stemming from fossil fuel consumption, many researchers have attempted to minimize the harmful effects induced by consuming fossil fuels in the electricity production, including pollutant production management and using new power generation technologies such as renewable energy sources and fuel cell technologies with greatly reduced pollutants.

The fuel cell as an emerging technology has received a great deal of attention over recent years, with each of the top industrial owners and governments in the world taking a special approach to fuel cell technology. A fuel cell is an electrochemical device that converts the chemical energy of a fuel directly into electrical energy without the Carnot cycle limitations.⁶ This technology includes some advantages compared to other technologies, such as lower environmental pollution, high-efficiency conversion, power generation ranging from less than few watts to multiple megawatts, application in different industries and transportation, lack of moving parts, lower maintenance costs, and lower noise pollution. Among the different fuel cells, SOFCs with a high potential for medium- and large-scale cogeneration allow direct use of light hydrocarbons (such as natural gas). Usage of fuel cells in large residential buildings may reduce maintenance costs of power-related equipment from 20% to 40% and increase efficiency by up to 85%. Figure 2 displays the emission rate of CO₂ pollutants in the fuel cells, internal combustion engines, and gas turbines.⁶ Based on the figure, the emission rates of pollutants, especially NOx, in fuel cells is lower than in other converters.

Figure 2.

Comparison of pollutants produced in gasoline and diesel engines and gas turbines with fuel cells.⁶

Although due to the low temperature of electrochemical reactions in fuel cells, the use of hydrogen and methanol does not result in the production of carbon dioxide and nitrogen oxides, the use of fossil fuels in fuel cells results in the production of carbon dioxide and sulfur dioxide, though its value is lower than that of power plants and internal combustion engines.⁶

Review of research on cogeneration systems indicates that in the most cases, the systems studied have been investigated regarding the first and second laws of thermodynamics while the environmental aspects of using fuel cell technology in power generation systems have not received much attention.^7–10 Only some references have briefly referred to the environmental issues in the cogeneration systems, some of which are outlined here.

Rahimi and Morteza investigated a hybrid system based on solid oxide fuel cell to generate power and heat using biomass fuel from thermodynamic and environmental impact point of view. In this study, the environmental impacts of the system were compared in three cases, including the power generation system, heat and power generation system, and trigeneration (power, heating, and cooling) system. The results showed that CO₂ production in the trigeneration system was 34.62% lower than the power generation system.¹¹ Singh et al.¹² proposed an integrated biomass-SOFC system to improve efficiency and minimize pollutants, where the carbon deposition induced by the fuel from the biomass gasifier and its adverse effects on the SOFC electrodes (anode) were addressed. A total of 32 different modes of the cycle were studied. The results indicated that the carbon deposition was minimized when the fuel cell was at the highest operating temperature, but when high-carbon fuel was used, carbon deposition increased with temperature rise and then reached a constant value.

Anastasiadis et al.¹³ performed a study on the economic and environmental effects of a microgrid composed of fuel cells, microturbines, and photovoltaic units. In this system, the fuel cell used natural gas as fuel, which has less environmental impact. Four different scenarios were considered in this regard. In the first scenario as a basic case, there was no fuel cell and microturbines. In the second scenario, fuel cells and microturbines were applied to meet the regional needs. The third scenario was the same as the second scenario except that the production of pollutants was optimized. In the fourth scenario, generating electricity and selling it to the grid were also considered. The results indicated that pollutant production in the fourth scenario was less than in the other three scenarios.

Najafi et al.¹⁴ analyzed thermodynamic, economical, and environmental (emissions cost) modeling of a SOFC–GT hybrid system integrated with a multi-effect flash (MSF) desalination unit. The exergetic efficiency was maximized and the total cost rate of the system was minimized, including the cost rate of environmental impact. A multi-objective genetic algorithm (MOGA) and a single-objective genetic algorithm (SOGA) were applied in order to determine the optimal design parameters of the plant. The results highlighted that use of MOGA results in maximized exergy efficiency, as well as minimized total system costs compared to SOGA.

Radmnanesh¹⁵ dealt with modeling and evaluating the technical, economic, and environmental performance of molten carbonate fuel cells compared to microturbine gas for the simultaneous production of electricity and heat. Their findings showed that the amount of CO₂ produced in a fuel cell and microturbine was 377 and 625 grams per kilowatt-hour representing 40% lower emissions in fuel cells, respectively. Due to rising carbon taxes in the coming years and lower fuel cell technology costs, they are predicting that the technology would be competitive in the future against conventional power generation technologies such as gas turbines.

Chatrattanawet et al.¹⁶ investigated a biogas solid oxide fuel cell system with two different reforming approaches (internal reforming and external reforming) to determine the optimum operating point. Their simulation results showed that both approaches had the same optimal operating point, including temperature 1173 K, pressure 3 atm, and current density 5000 A/cm². Under identical operating conditions, the internal reforming had a better electrical efficiency than the external reforming. Consideration of CO₂ and CO emissions from the anode side indicated that the values of these gases were higher in the internal reforming than in the external reforming.

Owebor et al.¹⁷ performed an analysis from energy, exergy, environmental, and economic viewpoints for an integrated power plant which applied municipal solid waste-resulted fuel. The proposed waste-to-energy conversion system was composed of gasification, SOFC, gas turbine, steam turbine, organic Rankine cycle (ORC), and absorption refrigeration cycle. They examined the effects of adiabatic flame temperature on the production rate of pollutants. They found that the NOx and CO emission rates were 1.009 and $8.1 \times 10^{- 6}$ kg/s at the system design point, respectively, where elevation of the adiabatic flame temperature increased the emission rates of these pollutants. Roy et al.¹⁸ analyzed an integrated system composed of a biomass gasification system, an SOFC module, an externally fired gas turbine, and ORC. Their environmental assessment results indicated that the maximum CO₂ emission rate reduction potential in the study system was computed to be 3564 t/year compared to the fossil fuel-based power plant.

Valencia et al.¹⁹ examined an integrated power generation system composed of a wind turbine, AIR 403, a 200 MW proton exchange fuel cell (PEM), an electrolyze, a solar panel, and a charge regulator based on the PID controllers in the Colombian Caribbean region. The purpose of this study was to improve the system for enhancing the efficiency and reducing environmental impacts to minimize energy costs and CO₂ emissions. The results from one case study highlighted that a high amount of CO₂ is released into the atmosphere because of using a large number of solar panels. It was also found that the emission rate of pollutants does not necessarily decrease as the number of solar panels drops.

Hybrid systems are designed and developed for different purposes, thus, not only the increased electrical power generation is of great importance, but also excess heat can be used in the industrial processes for heating and cooling, freshwater production, etc. Obviously, there are different approaches to producing freshwater in which electricity, heat, or both are employed. Lisbona et al.²⁰ and Eveloy et al.²¹ investigated a configuration of a power production system composed of a fuel cell and a reverse osmosis (RO) desalination plant, in which part of the produced electricity was used for freshwater production in the reverse osmosis units. In some other related works, the freshwater production has been studied using multiple-effect flash evaporation (MSF) and multiple effect distillation (MED) with sudden pressure drop. Indeed, in such systems which use waste heat resulting from the power generation cycle, the generated steam is consumed for freshwater production in the operation of evaporation desalination. Ghirardo et al.²² studied a heat-recovery system from thermodynamic and economic viewpoints in a marine ship for freshwater production using MSF and power generation using an ORC in order to reduce adverse environmental impacts of heat generated in the 250 kW SOFC fueled by methanol. They concluded that about 181 kW of generated heat can be recovered in an ORC to generate 35 kW of electricity, where 1.7 m³/h fresh water can be produced through recovering 212 kW of the generated heat.

Reyhani et al.²³ conducted a study from thermodynamic and economic points of view in which a system composed of an SOFC, a gas turbine, and a 100 kg/h gasification unit was first integrated with a steam cycle and then with a MED. The purpose of this study was to propose an energy-efficient system based on the fuel cell and heavy fuel oil gasification to reduce greenhouse gas emissions and power-fresh water cogeneration with respect to the Iran situation.

Ahmadi et al.²⁴ studied a 39 MW SOFT-GT-MED hybrid system for freshwater production in Qeshm, from thermodynamic and economical viewpoints. The system was optimized using a genetic algorithm with two objective functions to enhance the exergy efficiency and reduce the levelized cost of electricity.

Hosseini et al.²⁵ proposed an integrated SOFC and micro gas turbine (MGT) with a multi-effect desalination (MED) system from energy and exergy viewpoints. Based on their findings, the steam generator significantly affected the increasing production capacity of the desalination system.

Beyrami et al.²⁶ dealt with the modeling and multi-objective thermoeconomic optimization of a single effect desalination unit integrated with an SOFC system using the genetic algorithm. Three scenarios were considered in the optimization: 1) total exergy efficiency and total cost rate of hybrid system were assumed as a two-objective function, 2) the CO₂ emission rate and total cost rate of hybrid system were considered as two objectives, 3) the total exergy efficiency and the CO₂ emission rate were presumed as the optimization functions. They concluded that the fuel consumption coefficients within a range from 0.8 to 0.9, and the steam to carbon ratio within 2–2.5 significantly affected the optimization process. The minimal CO₂ emission rate was 0.05832 kg/s.

As can be seen, the integrated power generation systems composed of SOFC, gas turbine, and evaporation desalination have not been considerably addressed from an environmental viewpoint. Hence, it this study a mentioned integrated syatem have been analyzed due to the destructive impacts of biological pollutions, such as CO2, CO, and NOx. The system is first studied regarding the first law of thermodynamic, then; the impacts of some parameters including fuel pre-reforming percentage, the steam to carbon ratio, system operating pressure, and air-fuel ratio are examined on these species emissions.

System description

Figure 3 provides a schematic representation of the study system. Based on the figure, the fuel and oxidizer (air) are injected into the compressor from points 1 and 4 and are elevated up to the operating pressure of the system, respectively. Since the preheating of fuel and air fed into the fuel cell results in a severe decline in a thermal gradient in the fuel cell and improve its performance, the compressed air from compressors enters heat exchangers for preheating. The rate of air entering fuel cell is higher than the rate of fuel entering fuel cell; thus, the air is first preheated which requires more preheating than fuel. The concerned SOFC includes an internal reforming; thus, pre-reforming is essential to prevent carbon deposition, increased efficiency, and severe thermal gradients in the fuel cell, where the steam to carbon ratio should be at least 1.6 to avoid carbon deposition. Thus, at the beginning of operation and with respect to the steam to carbon ratio, the steam is injected into the system from point 27 and its control valve is closed immediately. The injected steam is continuously re-circulated through the system from point 11 and its pressure drop is compensated using a blower. The fuel injected into a fuel cell stack is not fully consumed, so an afterburner chamber is used to avoid wasting fuel. The pre-reforming process is an endothermic reaction, so part of the combustion chamber output is sent to the pre-reformer while the rest is sent to the turbine to provide the reaction with the required heat. The output mixture is injected into the air and fuel preheaters from point 20 and is then injected into the steam generator to generate steam.

Figure 3.

A schematic representation of hybrid system.

Thermodynamic analysis

In this section, a thermodynamic model is proposed according to the first law of thermodynamics for all components of the system. The required equations are described in short. For more details, see Reference.²⁷

The assumptions below are considered to simplify the thermodynamic model:

The exhaust gas mixtures from fuel cells are in chemical equilibrium.

The exhaust gases from the anode and cathode sides of fuel cell stock are isothermal.

A constant pressure drop is assumed for the components.

All system components are in a steady state.

Potential and kinetic energy changes in the components are ignored.

The input and output gases of a fuel cell are assumed ideal.

All system components are adiabatic, excluding the afterburner chamber and fuel cell stock.

The equations for conservation of mass and conservation of energy for a steady state system are expressed as follows:

\sum {\dot{m}}_{i} = \sum {\dot{m}}_{o}

(1)

\dot{Q} + \sum {\dot{m}}_{i} h_{i} = \dot{W} + \sum {\dot{m}}_{o} h_{o}

(2)

Fuel cell stock

Hydrogen is considered as an ideal fuel for all fuel cells due to the simple kinetics of hydrogen reaction at the anode; however, less pure hydrogen is available and thus it should be provided by reforming other fuels. Since SOFCs have a high operating temperature, in these cells it is possible to convert the light hydrocarbons into hydrogen-rich fuels. The fuel used in the system is natural gas which can be reformed internally or externally and then converted into a hydrogen-rich fuel. Here, the internal reforming is considered in order to reduce system costs. In addition, the tubular SOFC is also used which offers some advantages compared to a plate-type SOFC, including good sealing, higher energy efficiency, and high scale power generation.²⁸

In the internal reforming of fuel, the heat released from electrochemical reactions occurring in the catalysts is used to perform a reforming endothermic reaction. The reactions corresponding to methane-to-hydrogen-rich fuel conversion involve the steam-methane reforming reaction and water-gas shift reaction (WGSR), which are defined as follows:²⁹

C H_{4} + H_{2} O \to 3 H_{2} + C O

(3)

C O + H_{2} O \leftrightarrow 3 O_{2} + H_{2}

(4)

Hydrogen in the reformed fuel involves some reactions in the fuel cell causing generation of electricity, water, and heat. In the cathode catalyst, the oxygen molecules react with the electrons, producing oxygen ions according to the following reaction:³⁰

{\frac{1}{2} O}_{2} + e^{2 -} \to O^{2 -}

(5)

In addition, in the anode catalyst, the hydrogen molecules react with oxygen ions transferred from the cathode catalyst, producing water and electron according to the following reaction:³⁰

H_{2} + O^{2 -} \to H_{2} O + 2 e^{-}

(6)

Thus, the overall reaction of the fuel cell is given by:

H_{2} + {\frac{1}{2} O}_{2} \to H_{2} O

(7)

Considering $X_{r}$ , $Y_{r}$ and $Z_{r}$ respectively as the molar rates of progress of the steam-methane reforming reaction, WGSR and the overall reaction of the fuel cell, the fuel utilization ( $U_{f}$ ) will be determined as follows:²⁹

U_{f} = \frac{Z_{r}}{3 X_{r} + Y_{r}}

(8)

The first step to model the fuel cell is to determine its reversible voltage. The output voltage which varies from 0.6 V to 0.7 V, is less than the reversible voltage because of losses in the fuel cell. In this range, there is an equilibrium between fuel cell efficiency, power density, initial costs, and stable operation conditions, preventing anode oxidation at lower voltages. For the overall fuel cell reaction, the reversible voltage is calculated by the Nernst equation as follows:³¹

E = E^{°} + \frac{R_{u} T}{n_{e} F} \ln (P_{H_{2}} \times P_{O_{2}}^{\frac{1}{2}} / P_{H_{2} O})

(9)

Where, $E^{°}$ is a fuel cell voltage at standard pressure, $R_{u}$ universal Gas constant, T represents the temperature of the fuel cell stack, P is the partial pressure and $n_{e}$ is the number of electrons flowing in the outer circuit per hydrogen mole formation. The fuel cell voltage is a function of temperature at the standard pressure:²⁸

E^{°} = 1.272 - (T \times 2.764 \times 10^{- 4})

(10)

A limited current is extracted from the cell through connecting the external load to the fuel cell, which is an irreversible process causing deviation of the output voltage from the reversible voltage. There are three main factors affecting the voltage drop, including activation voltage drop, Ohmic voltage drop, and concentration voltage drop with all depending on the current density.³¹

V_{cell} = E - (V_{act} + V_{ohm} + V_{con})

(11)

The activation voltage drop is the sum of activation voltage drops in the anode and the cathode of fuel cell which are defined as follows by simplifying the Butler-Volmer equation:32

V_{act} = V_{act, a n} + V_{act, c a}

(12)

V_{act, a n} = \frac{R_{u} T}{F} \sinh^{- 1} (\frac{j}{{2 j}_{0, a n}})

(13)

V_{act, a n} = \frac{R_{u} T}{F} \sinh^{- 1} (\frac{j}{{2 j}_{0, c a}})

(14)

where,

j_{0, a n}

and

j_{0, c a}

are calculated by the following equations:

j_{0, a n} = γ_{a n} (\frac{P_{H_{2}}}{P_{ref}}) (\frac{P_{H_{2} O}}{P_{ref}}) \exp (- \frac{E_{act, a n}}{R_{u} T})

(15)

j_{0, c a} = γ_{c a} {(\frac{P_{o_{2}}}{P_{ref}})}^{0.25} e xp (- \frac{E_{act, c a}}{R_{u} T})

(16)

where, parameter

γ

depends on the material of anode and cathode electrodes while

E_{act}

is the activation energy.³² The resistance induced by electron movements in the anode and cathode electrodes, bipolar plates, as well as ion movements in the electrolyte leads to an ohmic voltage drop. Accordingly, the ohmic voltage drop for the four components is calculated by applying the following equations:³²

V_{ohm} = V_{ohm, a n} + V_{ohm, c a} + V_{ohm, e l} + V_{ohm, int}

(17)

V_{ohm} = j \sum_{k} r_{k}

(18)

r_{k} = ρ_{k} . δ_{k}

(19)

ρ_{k} = a_{k} . \exp (\frac{b_{k}}{T})

(20)

where a, b, and δ are the constant parameters which depend on the type and shape of fuel cells.

When a large current is extracted from the fuel cell, the current generation rate is not in accordance with the mass transfer rate, causing a severe voltage drop in the fuel cell. Thus, the concentration voltage drop is of great importance at high current densities, which is given by:³²

V_{con} = \frac{R_{u} T}{n_{e} F} \ln (1 - \frac{j}{j_{L}})

(21)

where,

j_{L}

is the limiting current density. The actual fuel cell voltage is determined by calculating the mentioned voltage drops and subtracting them from the thermodynamic voltage. The total power of fuel cell stack is determined using the following equation:³¹

W_{stack, d c} = V_{cell} . I_{stack}

(22)

where,

I_{stack}

is the stack output current and

W_{stack, d c}

is the stack output power.

The amount of waste heat is calculated using a thermal energy balance equation for the fuel cell:

\sum_{i} {\dot{n}}_{i} {(h_{f}^{°} + {\bar{h}}_{T} - {\bar{h}}_{298})}_{i} = \sum_{o} {\dot{n}}_{o} {(h_{f}^{°} + {\bar{h}}_{T} - {\bar{h}}_{298})}_{o} + {\dot{Q}}_{surr} + {\dot{W}}_{cell}

(23)

Afterburner chamber

A high fuel cell efficiency is achieved when the entire fuel and oxidizer injected into the fuel cells are not fully consumed and their residues are discharged along with other gases in the fuel cell channel. In order to enhance the efficiency of the fuel cell system and prevent energy losses, the anode and cathode exhaust gases enter the afterburner chamber, causing some reactions to generate some products. The temperature of products is calculated through the energy conservation equation for the combustion chamber as follows:³³

- {\dot{Q}}_{loss} - \sum_{P - R} (h_{f}^{°} + {\bar{h}}_{T} - {\bar{h}}_{298}) = 0

(24)

where,

{\dot{Q}}_{loss}

is the waste heat in the combustion chamber which depends on the efficiency of the combustion chamber and fuel heat value (calorific value):

{\dot{Q}}_{loss} = {\dot{n}}_{f} \times {LHV}_{f} \times (1 - η_{c c})

(25)

Turbine

The exhaust gases from the afterburner chamber can be used in the turbines because of specific features of high pressure and temperature. Considering the ideal power and isentropic efficiency, the electricity generation capacity of the turbine is calculated as follows:³²

{\dot{W}}_{T} = \sum_{k} {({\dot{n}}_{k} {\bar{h}}_{k})}_{i n} - \sum_{k} {({\dot{n}}_{k} {\bar{h}}_{k})}_{out, a}

(26)

Oxidizer and fuel compressors

These systems operate under high pressures to reduce the components of the fuel cell system; thus, the injected fuel and oxidizers should be compressed up to the operating pressure of fuel cells using a compressor. Considering the ideal power and isentropic efficiency, the power consumption of the compressor is calculated as follows:³⁴

{\dot{W}}_{C} = \sum_{k} {({\dot{n}}_{k} {\bar{h}}_{k})}_{out, a} - \sum_{k} {({\dot{n}}_{k} {\bar{h}}_{k})}_{i n}

(27)

Heat exchanger and steam generator

In the turbine, the hot exhaust gases can be used to preheat the fuel and air injected into the fuel cell, as well as to generate the steam required to conduct pre-reforming or the desalination system operation. To this end, a gas/gas counter-flow heat exchanger is used to preheat the fuel and air, while as gas/liquid vertical heat exchanger (steam generator) is employed to generate steam. The relations required to model the gas/gas heat exchanger have been taken from.³⁵

According to Figure 4, the steam generator includes three main parts: an economizer, an evaporator, and a superheater. The pinch point, adjacent point, and temperature of exhaust gases (exhaust steam) should be defined flor modeling the steam generator using the equation in Reference.³⁵

Figure 4.

A schematic representation of steam generator.

Pre-reforming

The pre-reforming is generally a heat exchanger with a catalyst bed with the air/fuel mixture flowing in the inner wall and the working fluid flowing in the outer wall. The reaction type of pre-reforming is a steam-reforming reaction. Thus, the main reactions occurring in pre-reforming include steam-methane reforming and WGSR:³⁴

{a CH}_{4} + {b H}_{2} O \leftrightarrow a (1 - x_{con}) {CH}_{4} + 3 a {x_{con} H}_{2} + (b - a x_{con}) H_{2} O + a x_{con} C O

(28)

CO + H_{2} O \leftrightarrow {CO}_{2} + H_{2}

(29)

r_{s / c} = \frac{b}{a}

(30)

Where, $x_{con}$ , $r_{s / c}$ , a and b represent the pre-reforming percentage, steam to carbon ration, molar rate of fuel and molar rate of steam, respectively.

Given the importance of pre-reforming temperature, the exhaust temperature of the pre-reformer is considered as its operating parameter. The exhaust temperature of the working fluid from the pre-reformer is defined according to the energy balance:

\sum_{k} {({\dot{n}}_{k} {\bar{h}}_{k})}_{i n} + {\dot{Q}}_{s, P R} - {\dot{Q}}_{r, P R} = \sum_{k} {({\dot{n}}_{k} {\bar{h}}_{k})}_{out}

(31)

Multi-effect evaporation desalination plant

Based on Figure 5, the concerned desalination is of multi-effect evaporation type with parallel feeding composed of several evaporators, a condenser, and a steam ejector. The end condenser is responsible for removing excess heat from the system. First, water with a given flow rate and given temperature enters the condenser. Then, its temperature increases due to heat conduction induced by the steam inside the tubes, where part of it is removed from the system as cooling water while the rest is injected into different effects of desalination as feedwater. In the first effect, feedwater is sprayed on the evaporative tubes and part of it is vaporized because of heat conduction induced by the steam inside the pipes, which is supplied from an external heat source. The rest enters the next effect as wastewater. The generated steam is conducted to the second effect and flows through the second evaporative tubes as the heating steam of the second effect, with this process being constantly repeated at all effects.

Figure 5.

A schematic representation of six-stage evaporative desalination with steam ejector.

There is another component named the flash boxes in addition to the requirement described in steps 2-N. The condensed heating steam in the second effect evaporator tubes enters the flash chamber of the same effect, and part of it evaporates and enters the third effect evaporator tubes along with the heating steam, with this process repeating constantly at all effects.

When steam enters the steam ejector, some steam is sucked out of the last effect of desalination and enters ejector suction chamber at the pressure of P_ev. The two steams are instantaneously mixed and then the mixture enters a converging section of the ejector where the steam compressive energy converts into kinetic energy, and the steam velocity reaches supersonic speed with reduction of the cross-section. The steam enters a diverging section after passing through the throat whereby its pressure grows. Finally, the exhaust steam from the ejector enters the first effect of desalination at a pressure of P_s which is higher than P_ev and less than P_m.

The assumptions considered for analyzing the desalination system are as follows:

Desalination plants operate under steady state conditions.

Physical properties of different fluid streams are calculated based on their average inlet and outlet temperatures.

The steam formed at each stage is salt-free.

The heat transferred from desalination to the environment is negligible.

The heat load for all stages is the same.

With respect to the environmental considerations, modeling is performed such that the concentration of salt in final wastewater (salinity) is less than 70,000 ppm.³⁶

The salinity of wastewater removed from each stage is such that it includes the minimum amount of sediment.

The temperature difference between consecutive steps is assumed constant.

If the temperature of the heating steam is considered T_s and T_N for the first and last stages, respectively, then:

Δ T = (T_{S} - T_{N}) / N

(32)

T_{1} = T_{S} - Δ T T_{i + 1} = T_{i} - Δ T, i = 2, \dots, N

(33)

The mass balance of water and salt in the first step desalination and from the second to Nth steps follows equations (34) and (35), and equations (36) and (37), respectively:

B_{1} = F_{1} - D_{1}

(34)

x_{1} = \frac{F_{1}}{B_{1}} x_{f}

(35)

B_{i} = F_{1} + B_{i - 1} - D_{i}, i = 2, \dots, N

(36)

x_{i} = \frac{F_{1}}{B_{1}} x_{f} + \frac{B_{i - 1}}{B_{i}} x_{i - 1}, i = 2, \dots, N

(37)

Where, F, B, and D are saltwater, wastewater, and freshwater flow rates in kg/s, and x is salinity, with i indicating the step number, respectively.

The heating steam supplied through the power generation cycle in the first step desalination is calculated using equation (38). Also, the energy balance equation in the first step and second to Nth steps is expressed as equation (39):³⁶

Q = m_{s} (h_{s, out} - h_{s, i n}) = m_{f} c_{P} (T_{1} - T_{f}) + m_{d_{1}} λ_{ν_{1}}

(38)

m_{d_{i - 1}} λ_{ν_{i - 1}} = m_{f} c_{P} (T_{i} - T_{f}) + m_{d_{i}} λ_{ν_{i}}

(39)

Where, x is salinity, h is enthalpy in kj/kg. K, f, b, d, s correspond to feedwater, saltwater removed in each step, steam generated in each step condensed at the later step, and the first step heating steam, respectively.

The following equations can be written based on the definition of total evaporation and condensation heat transfer coefficients:³⁶

Q_{e} = U_{e} A_{e} Δ T_{i}

(40)

Q_{c} = U_{c} A_{c} L M {T D}_{e}

(41)

Where, LMTD is the logarithmic mean temperature difference for the condenser, e and c correspond to the end evaporation and condensation. The total evaporation and condensation heat transfer coefficients, the end condenser, and the modeling details have been adopted from Reference.³⁷ The relations provided in³⁸ have been used for steam ejector heat modeling.

System environmental analysis

The environmental problems induced by fossil fuel consumption lead researchers to investigate cogeneration systems from an environmental viewpoint. Accordingly, in the present work, the CO, CO₂, and NO_x emission rates have been investigated.

The experimental results indicate that the CO and NO_x emission rates are not significant due to the lower operating temperature of the fuel cell than the internal combustion engines; thus, it is assumed that CO and NO_x are produced in the afterburner chamber. It should be mentioned that the amount of CO₂ emission into the atmosphere is determined based on combustion equation in the combustion chamber. The production rate of CO and NOx in the combustion reaction of the afterburner chamber depends on the adiabatic flame temperature. Accordingly, the production rate of 1-gram gas per kilogram of injected fuel is calculated using the following equations:³⁹

m_{C O} = \frac{0.179 \times 10^{9} \times \exp (7800 / T_{p z})}{p^{2} τ {(Δ p / p)}^{0.5}}

(42)

m_{{N O}_{x}} = \frac{0.15 \times 10^{16} \times τ^{0.5} \times \exp (- 71100 / T_{p z})}{p^{0.05} {(Δ p / p)}^{0.5}}

(43)

Where, P is the pressure at the burner chamber inlet, $Δ p / p$ denotes the pressure drop in the afterburner chamber, $τ$ represents the ignition timing assumed to be 0.002 seconds, and $T_{p z}$ is the adiabatic flame temperature calculated as follows:³⁹

T_{p z} = A σ^{α} e x p (β {(σ + λ)}^{2}) π^{x^{*}} θ^{y^{*}} ψ^{z^{*}}

(44)

Where, $π$ is a dimensionless pressure ( $Δ p / p$ ), θ denotes a dimensionless temperature ( $\frac{T}{T_{ref}}$ ) and $ψ$ shows the hydrogen to carbon ratio (H/C). If $ϕ \leq 1$ , then $σ = ϕ$ , and if $ϕ \geq 1$ , then $σ = ϕ - 0.7$ where $ϕ$ indicates the fuel-air equivalence ratio. The quantities $x^{*}$ , $y^{*}$ , and $z^{*}$ are calculated as follows:³⁹

x^{*} = a_{1} + b_{1} σ + c_{1} σ^{2}

(46)

y^{*} = a_{2} + b_{2} σ + c_{2} σ^{2}

(47)

z^{*} = a_{3} + b_{3} σ + c_{3} σ^{2}

(48)

Where, α, β, A, λ, a_i, b_i, and c_i are constants provided in detail in Table 1.

Table 1.

Constant parameters for equations (44) to (47).³⁹

	1 ≤ ϕ ≤ 0.3		1.6 ≤ ϕ ≤ 1
	2 ≤ θ ≤ 0.92	3.2 ≤ θ ≤ 2	2 ≤ θ ≤ 0.92	3.2 ≤ θ ≤ 2
$A$	261.7644	2315.752	916.8261	1246.1778
$α$	0.1157	–0.0493	0.2885	0.3819
$β$	–0.9489	–1.1141	0.1456	0.3479
$λ$	–1.0976	–1.1807	–3.2771	–2.0365
$a_{1}$	0.0143	0.0106	0.0311	0.0361
$b_{1}$	–0.0553	–0.045	–0.078	–0.085
$c_{1}$	0.0526	0.482	0.497	0.0517
$a_{2}$	0.3955	0.5688	0.254	0.0097
$b_{2}$	–0.4417	–0.55	0.2602	0.502
$c_{2}$	0.141	0.1319	–0.1318	–0.2471
$a_{3}$	0.0052	0.0108	0.0042	0.017
$b_{3}$	–0.1289	–0.1291	–0.1781	–0.1894
$c_{3}$	0.827	0.0845	0.098	0.1037

Social costs due to the production of pollutants include damage to human health and the environment, which leads to loss of welfare and increase external costs is calculated as follows:

{\dot{C}}_{env} = {\dot{m}}_{C O} c_{C O} + {\dot{m}}_{NOx} c_{NOx} + {\dot{m}}_{C O 2} c_{C O 2}

(48)

{\dot{m}}_{NOx}

{\dot{m}}_{C O}

, and

{\dot{m}}_{C O 2}

are the exhaust mass flow rates of nitrogen oxides, carbon monoxide, and carbon dioxide respectively, while

c_{NOx}

c_{C O}

, and

c_{C O 2}

are their corresponding damage unit costs and are assumed to be 6.853 US$/kg NO_x, 0.02086 US$/kg CO, and 0.0224 US$/kg CO₂, respectively.¹³

Results

A computational code has been written using Matlab to model an integrated system composed of SOFC, gas turbine, and multi-effect evaporation desalination plant, allowing for an energy and environmental analysis. The flowchart of hybrid system and METVC is depicted in Figure 6. Next, the system proposed by Pirkandi et al.⁴⁰ was simulated for the validation of results, with the related information summarized in Table 2. As well as, Table 3 shows the verification of METVC model. Based on the results, there is a good agreement between them.

Figure 6.

(a) Flowchart of hybrid system, (b) Flowchart of METVC.

Table 2.

Validation of hybrid system.

Parameter	Reference⁴⁰	Code
Current density (A/m²)	2641	2640
Turbine Inlet Temperature (TIT) (K)	1473	1484
Fuel cell stack temperature (K)	1161	1163
Net power output (kW)	960.7	986
Electrical efficiency	51.44	51.44

Table 3.

Verification of METVC model.

	n = 4		n = 5		n = 6
Parameter	Ref.⁴¹	Present model	Ref.⁴¹	Present model	Ref.⁴¹	Present model
D (kg/s)	0.7	0.7	0.88	0.89	1.08	1.10
m_sw (kg/s)	4.81	5.03	5.02	5.23	5.52	5.65
A_tot (m²)	222	219	283	279	355	361
PR	6.14	6.05	7.72	7.80	9.21	9.61

The parameters used to simulate the hybrid system and METVC are listed in Tables 4 and 5, respectively. The results from the hybrid system simulation are also presented in Table 6.

Table 4.

Hybrid system input parameters.

Parameter	Unit	Value
Input fuel rate⁴⁰	kmol/h	10
Input air rate⁴⁰	kmol/h	170
Steam to carbon ratio⁴⁰	–	2.5
Pre-reforming percentage⁴⁰	%	30
Compressor pressure rate⁴⁰	–	3
Number of cells⁴⁰	–	5133
Fuel cell active cross section⁴⁰	m²	0.1036
Dropped fuel cell pressure⁴⁰	%	4
Dropped combustion chamber pressure⁴⁰	%	5
Dropped recover pressure⁴⁰	%	4
Compressors isentropic efficiency⁴⁰	%	81
Gas-to-gas heat exchanger efficiency⁴⁰	%	85
Afterburner chamber efficiency⁴⁰	%	95
Generator efficiency⁴⁰	%	95
Inverter efficiency⁴⁰	%	89

Table 5.

METVC model input parameters.

Parameter	Unit	Value
Inlet steam temperature to the first stage³⁸	ºC	79
Temperature of the first stage³⁸	ºC	67
Feeding salt concentration³⁸	ppm	40000
Condenser temperature difference³⁸	–	10
Feeding water temperature³⁸	ºC	35

Table 6.

Hybrid system simulation results.

Output parameter	Unit	Value
System electrical efficiency	%	56.23
Total system efficiency	%	81.65
System exergy efficiency	%	60.7
Exergy destruction rate	kW	670.9
Heat recovery rate	kW	465
Cell voltage	V	0.68
Net power output	kW	1215
Current density	A/m²	3147

As simulation outputs, the characteristics of each stream of the hybrid system for the input parameters of the given model summarized in Table 7.

Table 7.

Characteristics of each stream of the hybrid system for the input parameters.

Stream	Pressure (bar)	Temprature (K)	Molar flow rate (kmol/h)	Percentage of components
Stream	Pressure (bar)	Temprature (K)	Molar flow rate (kmol/h)	CH₄	H₂	H₂O (g)	CO₂	CO	N₂	O₂	H₂O (L)
1	1.013	298	170	–	–	–	–	–	79	21	–
2	3.292	443.85	170	–	–	–	–	–	79	21	–
3	3.1606	1130.9	170	–	–	–	–	–	79	21	–
4	1.013	298	10	97	–	–	1.5	–	1.5	–	–
5	3.292	405.1	10	97	–	–	1.5	–	1.5	–	–
6	3.1606	695.37	10	97	–	–	1.5	–	1.5	–	–
7	3.1606	1075.7	34.25	28	–	71	0.5	–	0.5	–	–
8	3.1606	950	40.07	17	27	47	5.5	1.8	1.7	–	–
9	3.041	1314.1	154.53	–	–	–	–	–	87	13	–
10	3.041	1314.1	53.65	≫1	1.27	71	14.51	3.84	0.38	–	–
11	3.041	1314.1	24.25	–	–	100	–	–	–	–	–
12	3.1606	1314.1	24.25	–	–	100	–	–	–	–	–
13	3.041	1314.1	29.4	≫1	18.75	47.2	26.49	7	0.56	–	–
14	2.8824	1607.3	180	–	–	10.78	5.47	–	74.7	9.05	–
15	2.8824	1607.3	18	–	–	10.78	5.47	–	74.7	9.05	–
16	1.0992	1607.3	18	–	–	10.78	5.47	–	74.7	9.05	–
17	1.0992	1607.3	18	–	–	10.78	5.47	–	74.7	9.05	–
18	2.8824	1607.3	162	–	–	10.78	5.47	–	74.7	9.05	–
19	1.0992	1331.7	162	–	–	10.78	5.47	–	74.7	9.05	–
20	1.0992	1302.7	180	–	–	10.78	5.47	–	74.7	9.05	–
21	1.0552	767.93	180	–	–	10.78	5.47	–	74.7	9.05	–
22	1.013	747.86	180	–	–	10.78	5.47	–	74.7	9.05	–
23	1.013	403.21	180	–	–	10.78	5.47	–	74.7	9.05	–
24	1.013	298	33.79	–	–	–	–	–	–	–	100
25	10	300.03	33.79	–	–	–	–	–	–	–	100
26	10	722.86	33.79	–	–	100	–	–	–	–	–

The exergy destruction rates of cycle components are illustrated in Figure 7. As is shown in this figure, the highest amount of exergy destruction rate occurs in SOFC stack. Combustion chamber, METVC and pre-reformer have the next highest amount of exergy destruction rate, respectively.

Figure 7.

Exergy destruction rate in various components of the total system.

Of parameters influencing the emission rates of pollutants in the system, the impacts of compressor pressure rate, fuel consumption coefficients, and pre-reforming rate are investigated on the emission rates of different pollutants as well as on the social costs resulting from their emission.

Impacts of compressor pressure ratio

Figure 8 displays the impacts of the compressor pressure ratio on the CO, CO₂, and NOx emission rates. Based on the figure, the fuel cell exhaust temperature will drop as the compressor pressure ratio increases. Also, the CO and NOx emission rates are directly proportional to temperature, as a result, the emission rate of gases diminishes with elevation of the compressor pressure ratio. However, the way CO₂ changes with pressure changes is slightly different from the other two gases; the amount of the first two gases gradually declines and eventually converges to zero with increasing the pressure ratio, while CO₂ content drops down to the pressure ratio of 4 and increases thereafter.

Figure 8.

Changes of CO, CO₂, and NOx emission rates in system with respect to pressure.

Figure 9 depicts the impacts of the compressor pressure ratio on the social costs corresponding to the CO, CO₂, and NOx emission rates, as well as usual air-fuel ratios used in these systems. Since the social costs for the NOx emission rate are far higher than for other gases, changes in the emission rate of NOx significantly affect the final costs. In addition, at a constant input fuel flow rate, with elevation of the air-fuel ratio, the fuel cell exhaust temperature decreases, and the mentioned pollutant emission rates and corresponding social costs also diminish.

Figure 9.

Changes of social costs related to CO, CO₂, and NOx emission rates in system with respect to compressor pressure ratio.

Impacts of pre-reforming

Figure 10 indicates the changes occurring in the CO and NOx emission rates with respect to fuel pre-reforming percentage. Based on the figure, the pre-reforming percentage does not affect the CO and NOx emission rates considerably, while its impact on the CO₂ emission rate has been significant. The temperature of fuel cell exhaust gases increases as the pre-reforming percentage grows while the temperature gradient induced by internal reforming reaction diminishes, respectively. Hence, the CO and NOx emission rates increase with elevation of the pre-reforming percentage.

Figure 10.

Changes of CO, CO₂, and NOx emission rates in system with respect to pre-reforming percentage.

Figure 11 reveals the social cost changes induced by CO and NOx emission rates with respect to the pre-reforming percentage of fuel at different sulfur to carbon ratios. Based on the figure, the social costs related to the CO and NOx emission rates increase as the pre-reforming percentage grows. In addition, the CO and NOx emission rates and corresponding social costs increase as the steam to carbon ratio rises due to enhanced recovery and fuel cell's exhaust temperature.

Figure 11.

Social costs of CO, CO₂, and NOx emission rates with respect to pre-reforming percentage.

Impacts of fuel consumption coefficients

Figure 12 presents the changes occurring in CO and NOx emission rates with respect to fuel consumption coefficients. Based on the figure, the CO and NOx emission rates drop as the fuel consumption coefficients increase while the fuel cell's exhaust temperature falls. The changes in the fuel consumption coefficients do not affect the NOx emission rate significantly. The CO emission rate constantly drops as the fuel consumption coefficients grow, while the CO₂ emission rate falls down to a reforming rate of 0.92 and then rises.

Figure 12.

CO, CO₂, and NOx emission rates with respect to fuel consumption coefficients.

In addition, the corresponding social costs also decrease as their emission rates drop. Based on Figure 13, the corresponding social costs decline with increasing the fuel consumption coefficients, while the social costs grow with further increase in the fuel consumption coefficients. Due to increased NOx emission rate and the significant impact of changes in these species on the final costs. We found out that social costs reduce with increasing the number of cells and decreasing the fuel cell's exhaust temperature.

Figure 13.

Social costs for CO, CO₂, and NOx emission rates with respect to fuel consumption coefficients.

Conclusion

In this work, a fuel cell hybrid system was proposed and investigated based on design, including SOFC, GT, and multi-effect evaporation desalination plants. The results from the environmental analysis of the hybrid system can be summarized as follows:

Of different species, the impacts of CO, CO₂, and NOx emission rates were significant on the environment; thus, the impacts of pressure ratio and pre-reforming percentage on their emission rates were studied.

With increasing the compressor pressure ratio, the fuel cell's exhaust temperature, the CO, and NOx emission rates, and the social costs corresponding to the CO, CO₂, and NOx emission rates diminishes, while changes occurring in the NOx emission rate significantly affected the final costs.

With a constant flow rate of input fuel and increased air-fuel ratio, the fuel cell's exhaust temperature, species emission rates, and associated social costs diminished.

With increasing the pre-reforming percentage and reduction of the temperature gradient induced by fuel internal reforming reaction, the temperature of fuel cell exhaust gases rose.

With increasing the ratio of steam to carbon, the recovery rate, fuel cell's exhaust temperature, concerned gas emission rates, and corresponding social costs increased.

With elevation of the fuel consumption coefficients and reduction of the fuel cell's exhaust temperature, the CO and NOx emission rates and corresponding social costs decreased.

Footnotes

Declaration of conflicting interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) received no financial support for the research, authorship, and/or publication of this article.

ORCID iD

Hasan Hassanzade

Sobhan Jehandideh received his MsC in Mechanical Engineering -- Energy System Engineering from University of Birjand, Birjand, Iran in 2018. At present, he is working on the following research topics: the field of energy, Fuel cells and cogeneration cycles. He is also actively involved in energy-related industrial projects.

Hasan Hassanzade received his PhD in Mechanical Engineering from Shahid Bahonar University of Kerman in Iran in 2006 and is currently an associate professor in the Department of Mechanical Engineering at University of Birjand in Iran. He works in the field of fuel cell modeling.

Seyyed Ehsan Shakib received his PhD in Mechanical Engineering -- Energy System Engineering from the KN Toosi University of Technology, Tehran, Iran in 2012. At present, he is working on the following research topics: the field of energy and energy conversion, Desalination technology, renewable energy systems, and cogeneration cycles. He has contributed to 20 international papers. He is also actively involved in energy-related industrial and is an assistant professor of Bozorgmehr University of Qaenat.

Appendix

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